Hello there, naeshaA.
Sounds like you've done some great initial troubleshooting. When an app on your iOS device is unresponsive, this Knowledge Base article offers some great starting points for troubleshooting as well:
If an app you installed unexpectedly quits, stops responding, or won’t open - Apple Support
Since Mail is an iOS native app, these steps are the ones that apply in your case:
Follow these steps if you have an unresponsive app on your iPhone, iPad, or iPod touch.
Thanks for participating in Apple Support Communities.
Kind Regards,
Pedro.